1. Field of the Invention
The present invention relates to fluorinated compounds that may be polymerized to form materials exhibiting low light loss, and to planar waveguide devices fabricated from polymerizable fluorinated compounds.
2. Technical Background
It is known to fabricate planar waveguide devices such as optical switches, variable attenuators, and tunable gratings of polymeric materials. Polymeric waveguide devices generally comprise a lower cladding layer having a low refractive index, a core layer having a high refractive index and an upper cladding layer having a low refractive index.
A typical method of fabricating waveguide components of polymeric materials involves depositing a liquid polymerizable composition onto the surface of a substrate, such as by spin-coating, and subsequently polymerizing the deposited coating. Fluorinated acrylate liquid monomers have the advantages of being solvent-free and provide low loss waveguide structures with very low birefringence. Conventional polymerizable coating compositions of this type, however, generally have several disadvantages. A significant disadvantage with typical compositions is that the polymerized material usually has a refractive index that is substantially lower than silica. This results in a mismatch between the refractive index of the polymer waveguide of the planar optical device and the silica waveguide of an optical fiber connected to the polymer waveguide. This mismatch of the refractive indexes increases coupling losses at the interface between the waveguides, and can also lead to unwanted reflection of a guiding optical signal at the interfaces of waveguide and fiber. The relatively low refractive index of typical polymers used in planar optical components is caused by the use of fluorinated monomers. The fluorinated monomers can be used to reduce the volume density of carbon-hydrogen bonds in polymeric optical materials and thereby reduce absorption losses associated with the vibration modes of these carbon-hydrogen bonds. Thus, although coupling losses between silica optical fibers and polymer waveguides can be reduced by lowering the concentration of fluorinated monomers, this would cause an undesirable increase in absorption losses. Therefore, it would be desirable to provide polymeric coating compositions that can be polymerized to form optical components having a refractive index more closely matched to the refractive index of silica, without increasing absorption losses.
Another disadvantage with conventional coating compositions used to form polymeric optical components is that the viscosity of such compositions is undesirably low, typically less than 100 centipoise. This low viscosity imposes constraints on the thickness of a single layer that can be applied by spin-coating. In addition, very low viscosity materials will sometimes flow along and/or from the surface of a substrate or previously spin-coated layer resulting in uneven coating thickness. This is undesirable because planar waveguides comprised of layers having a non-uniform thickness exhibit increased loss of light and other undesirable optical behavior. Therefore, polymerizable compositions having a relatively higher viscosity would be desirable for fabrication of planar waveguide devices.
Another problem with conventional polymerizable compositions used to fabricate planar waveguide devices is that the lower molecular weight monomers typically used in such compositions exhibit significant volatility, especially when the composition is exposed to a high vacuum. Typical polymerizable compositions used for fabricating planar waveguide devices include a monomer blend in which the various monomer components in the blend have different relative volatilities. As a result, the concentration of the various monomer components can change during processing steps, such as spin coating and polymerization. Because relatively thin layers (e.g., typically from about 2 microns to about 10 microns) are deposited during fabrication of a planar waveguide device, the compositional changes caused by the different relative volatilities of the monomer components can be relatively severe, profoundly degrading the ability to control the refractive index of the polymerized material. It is often desirable to polymerize the coating compositions under a vacuum or under a nitrogen blanket to eliminate the presence of oxygen during polymerization. In such cases, even a relatively low volatility makes it especially difficult to control the processes necessary to ensure that the coating has the composition required to achieve a desired refractive index. Volatile monomers also adversely affect the ability to use proximity printing processes (photolithographic processes in which a mask is spaced from the substrate during the radiation exposure step), since the volatile monomers can fog the mask during the printing process.
Accordingly, it would be highly desirable to provide polymerizable compositions for fabricating planar waveguide devices that exhibit a more favorable range of viscosities, with a lower volatility, and which can be polymerized to form a waveguide material that exhibits low light absorption losses and which has a refractive index that is closer to the refractive index of a silica optical fiber. More specifically, it would be desirable to provide fluorinated polymerizable compounds that exhibit a selected range of viscosities, low volatility, and that can be polymerized to form a material having a refractive index that more closely matches the refractive index of optical fibers.
The invention provides fluorinated compounds that overcome problems associated with conventional compositions used for fabricating polymeric waveguides. More specifically, the invention provides fluorinated compounds that may be polymerized to form materials exhibiting a higher refractive index that is more closely matched to that of a silica waveguide, while also achieving low absorption losses. The fluorinated compounds may also be used in coating compositions that have higher viscosities that are favorable for fabricating relatively thick layers of uniform thickness. The fluorinated compounds of this invention may also be used for preparing coating compositions having a relatively low volatility, whereby the fabrication of devices having a desired refractive index is more easily controllable, and which facilitates proximity printing.
In accordance with an aspect of the invention, the fluorinated compound comprises at least one fluorinated alkylene or alkylene ether moiety, and at least two terminal acrylate moieties. Each of the terminal acrylate moieties is connected to the fluorinated alkylene or fluorinated ether moiety or moieties by an ester (xe2x80x94CO2xe2x80x94) linking group.
The invention also pertains to polymerizable compositions containing the fluorinated compounds, and to planar waveguide devices fabricated from such compositions.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawing is included to provide a further understanding of the invention and is incorporated and constitutes part of this specification. The drawing illustrates various features and embodiments of the invention, which, together with their description serve to explain the principals and operation of the invention.